When I fetch the data for the first time everything works fine and I am able to see the data on the screen, the data is also inserted in the sqlite database, but when I reopen the app I get an error sayingflutter: The following NoSuchMethodError was thrown building StreamBuilder<StudentModel>(dirty, state:flutter: _StreamBuilderBaseState<StudentModel, AsyncSnapshot<StudentModel>>#3f888):flutter: The getter 'studentData' was called on null.flutter: Receiver: nullflutter: Tried calling: studentData这是我的模特班class StudentModel { int status; String msg; StudentModelData studentModelData; StudentModel({this.status, this.msg, this.studentModelData}); StudentModel.fromJson(Map<String, dynamic> json) { status = json['status']; msg = json['msg']; studentModelData = json['data'] != null ? new StudentModelData.fromJson(json['data']) : null; } StudentModel.fromDb(Map<String, dynamic> parsedJson) { status = parsedJson['status']; msg = parsedJson['msg']; studentModelData = parsedJson['data'] != null ? new StudentModelData.fromJson(parsedJson['data']) : null; } Map<String, dynamic> toJson() { final Map<String, dynamic> data = new Map<String, dynamic>(); data['status'] = this.status; data['msg'] = this.msg; if (this.studentModelData != null) { data['data'] = this.studentModelData.toJson(); } return data; }}class StudentModelData { int lastIndex; List<StudentData> studentData; StudentModelData({this.lastIndex, this.studentData}); StudentModelData.fromJson(Map<String, dynamic> json) { lastIndex = json['lastIndex']; if (json['studentData'] != null) { studentData = new List<StudentData>(); json['studentData'].forEach((v) { studentData.add(new StudentData.fromJson(v)); }); } } Map<String, dynamic> toJson() { final Map<String, dynamic> data = new Map<String, dynamic>(); data['lastIndex'] = this.lastIndex; if (this.studentData != null) { data['studentData'] = this.studentData.map((v) => v.toJson()).toList(); } return data; }}class StudentData { String studentId; String studName; String studProfilepic; String studentEmail; String studentMobile; String courseName; String classCode; int minAvg; int avg; StudentData( {this.studentId, this.studName, this.studProfilepic, this.studentEmail, this.studentMobile, this.courseName, this.classCode, this.minAvg, this.avg}); StudentData.fromJson(Map<String, dynamic> json) { studentId = json['student_id']; studName = json['stud_name']; studProfilepic = json['stud_profilepic']; studentEmail = json['student_email']; studentMobile = json['student_mobile']; courseName = json['course_name']; classCode = json['class_code']; minAvg = json['minAvg']; avg = json['avg']; } StudentData.fromDb(Map<String, dynamic> parsedJson){ studentId = parsedJson['student_id']; studName = parsedJson['stud_name']; studProfilepic = parsedJson['stud_profilepic']; studentEmail = parsedJson['student_email']; studentMobile = parsedJson['student_mobile']; courseName = parsedJson['course_name']; classCode = parsedJson['class_code']; minAvg = parsedJson['minAvg']; avg = parsedJson['avg']; } Map<String, dynamic> toJson() { final Map<String, dynamic> data = new Map<String, dynamic>(); data['student_id'] = this.studentId; data['stud_name'] = this.studName; data['stud_profilepic'] = this.studProfilepic; data['student_email'] = this.studentEmail; data['student_mobile'] = this.studentMobile; data['course_name'] = this.courseName; data['class_code'] = this.classCode; data['minAvg'] = this.minAvg; data['avg'] = this.avg; return data; }}以下是我的存储库类class StudentDbProvider implements Source, Cache { Database db; void init() async { print("database initialized"); Directory documentsDirectory = await getApplicationDocumentsDirectory(); final path = join(documentsDirectory.path, "students.db"); db = await openDatabase(path, version: 1, onCreate: (Database newDb, int version) { newDb.execute(""" CREATE TABLE STUDENTS( id INTEGER PRIMARY KEY, student_id TEXT, stud_name TEXT, stud_profilepic TEXT, student_email TEXT, student_mobile TEXT, course_name TEXT, class_code TEXT, minAvg TEXT, avg TEXT ) """); }); } @override Future<int> clear() { return db.delete("STUDENTS"); } @override Future<StudentModel> fetchStudents(String disciplineId, String schoolId, String year_id, String lastIndex) async { print("PritishSawant${db==null}"); final maps = await db.query("STUDENTS"); if (maps.length > 0) { return StudentModel.fromDb(maps.first); } return null; } @override Future<int> addStudent(StudentData studentData) { return db.insert("STUDENTS", studentData.toJson(), conflictAlgorithm: ConflictAlgorithm.ignore); }}final studentDbProvider = StudentDbProvider();以下是我的集团课程class StudentsBloc { final _repository = Repository(); final _students = PublishSubject<StudentModel>(); Observable<StudentModel> get students => _students.stream; fetchStudents(String disciplineId,String schoolId,String year_id,String lastIndex) async { await studentDbProvider.init(); final student = await _repository.fetchStudents(disciplineId, schoolId, year_id, lastIndex); _students.sink.add(student); } clearCache(){ return _repository.clearCache(); } dispose(){ _students.close(); }}据我所知，错误一定是由于数据库初始化不正确引起的，但是当我执行第一个网络请求时，一切工作正常，并且控制台中没有出现任何错误，并且数据库也已初始化.我不明白为什么第二次以后会发生错误?As far as I can understand the error must be occurring due to improper database initialisation but when I did the first network request everything was working fine and I did not get any error in the console and the database was also initialised. I am not able to understand why the error is occurring for the second time onwards?推荐答案我想您正在代码的某些部分调用snapshot.data.studentData.I suppose you are calling snapshot.data.studentData in some part of your code.在流构建器上，我倾向于首先进行空检查On a stream builder, I tend to first do a null checkif (snapshot.data != null) { // your code here}然后继续验证数据，否则您可以在StreamBuilderAnd then proceed to verify the data, else you can use a getter on a null data provided by the StreamBuilder 这篇关于StreamBuilder抛出脏状态的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！ 上岸，阿里云！
